In solid lumber construction, which section of the span is restricted from having notches?

In solid lumber construction, the restriction on notching primarily affects the middle third of the span. This section is considered critical because it is where the bending stress is highest when the beam or joist is loaded. Notching in this area can significantly weaken the structural integrity of the lumber, potentially leading to failure under load.

The reasoning behind this restriction is rooted in how loads are distributed along the span of a beam. When a load is applied, the middle section experiences greater tension and compression, making it more susceptible to failure if notches are present. The structural principles that govern beam design dictate that notches should be avoided where stress is concentrated to maintain safety and performance.

In contrast, notches in the outer third or at the ends may be permissible under certain conditions as these sections experience lower stress levels. However, even in those areas, care must be taken to ensure that notching does not compromise the overall strength or function of the lumber.